Overview

A process experiment is beginning in the US under NSF sponsorship concentrating
on the formation, evolution, storage, dispersal and large-scale consequences
of Eighteen Degree Water, the subtropical mode water of the North Atlantic.
Mode waters are a physical manifestation of air–sea exchange that, through
successive wintertime exposure, constitute a long-term memory within the climate
system. Present understanding of the annual renewal rate of these waters and
the responsible physical mechanisms is deficient; water mass transformations
inferred from climatological air–sea fluxes appear incompatible with both
volumetric analyses and estimated dissipation processes. It is hypothesized
that current formation rate estimates are inaccurate due to (i) poorly estimated
air–sea fluxes and (ii) poorly understood/represented lateral ocean processes.
Through a synthesis of ocean-atmosphere observations and modeling studies,
the CLIMODE program (CLIvar MOde water Dynamics Experiment) will:

Investigate processes common to all subtropical mode waters in the world
ocean, which are found equatorward of strong zonal flows (in particular
the Gulf Stream, the Kuroshio Extension, the East Australian Current, the
Agulhas Return Current and the Antarctic Circumpolar Current).

Focus on a region of great importance in air–sea exchange –– huge
ocean to atmosphere heat loss occurs over the separated Gulf Stream where
uncertainties in flux estimates are large.

Explore the cross-scale connection between water mass formation associated
with wintertime convection on the rim of the subtropical gyre, its subduction
into the stratified interior, and its dispersal and dissipation around the
gyre.

Address a key process that is presently poorly understood and inadequately
represented in climate models –– the interaction of geostrophic eddies
with mixed layers. This is one of the most critical parameterizations that
compromises the present generation of climate models. CLIMODE will provide
field observations and analysis of the interaction of eddies and the mixed
layer, in support of the CLIVAR Climate Process Team in ocean mixing responsible
for developing/testing parameterizations.

CLIMODE will have broad scientific impact because it directly addresses
oceanic phenomena and atmosphere–ocean coupling that have climatic significance
but which are inadequately represented in climate models. Furthermore, through
assessment/validation of the climate model parameterizations of these processes,
CLIMODE will have considerable societal impact. The program will also contribute
to the career advancement of several graduate students and postdoctoral investigators
who will participate in the study. On average during the 5 year program, CLIMODE
will directly support 5 students and 4 to 5 postdocs each year.

CLIMODE 2: Data Analysis Phase

The analysis phase of CLIMODE has begun with NSF sponsorship. In this phase,
we will examine data collected in CLIMODE for Eighteen Degree Water (EDW)
dynamics. Our original breakdown of the science has evolved - now we find
that air-sea interaction is part of all three new scientific groupings as
this is a critical forcing function of EDW formation. Our present science
groups reflect the need for a broad view of the study region over the extended
CLIMODE period beginning in 2006 with our first measurements (group 1).
This will encompass the region of the entire northern subtropical gyre that
has been sampled by CLIMODE floats. Results from CLIMODE will be used to better
understand the longer and broader-scale satellite record of interannual variability
where we can see substantial changes over time (group 2). Group
3 will examine the frontal nature of EDW formation, where the mean vorticity
of the Gulf Stream and the strong surface forcing conspire to enable EDW formation
in a different mode than the rest of the Sargasso Sea. There is still an aspect
of the original air-sea interaction that is being carried forward by some
of PIs in the original scientific grouping of CLIMODE. At this stage, their
involvement will be sought in all of the above new scientific areas where
air-sea interaction plays a key role.

Evolution and Fate of EDW in the North Atlantic Subtropical Gyre

Dynamics of EDW from CLIMODE Observations and its Climate Implications